linux-hardening - Re: [PATCH][next] platform/chrome: Fix -Warray-bounds warnings

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Date: Thu, 14 Dec 2023 08:35:17 -0800
From: Kees Cook <keescook@...omium.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Benson Leung <bleung@...omium.org>, Guenter Roeck <groeck@...omium.org>,
	chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] platform/chrome: Fix -Warray-bounds warnings

On Wed, Mar 29, 2023 at 07:54:02PM -0600, Gustavo A. R. Silva wrote:
> GCC-13 (and Clang) does not like having a partially allocated object,
> since it cannot reason about it for bounds checking.
> 
> Notice that the compiler is legitimately complaining about accessing
> an object (params, in this case) for which not enough memory was
> allocated.
> 
> The object is of size 20 bytes:
> 
> struct ec_params_vbnvcontext {
> 	uint32_t                   op;                   /*     0     4 */
> 	uint8_t                    block[16];            /*     4    16 */
> 
> 	/* size: 20, cachelines: 1, members: 2 */
> 	/* last cacheline: 20 bytes */
> };
> 
> but only 16 bytes are allocated:
> 
> sizeof(struct ec_response_vbnvcontext) == 16
> 
> In this case, as only enough space for the op field is allocated,
> we can use an object of type uint32_t instead of a whole
> struct ec_params_vbnvcontext (for which not enough memory is
> allocated).
> 
> Fix the following warning seen under GCC 13:
> drivers/platform/chrome/cros_ec_vbc.c: In function ‘vboot_context_read’:
> drivers/platform/chrome/cros_ec_vbc.c:36:15: warning: array subscript ‘struct ec_params_vbnvcontext[1]’ is partly outside array bounds of ‘unsigned char[36]’ [-Warray-bounds=]
>    36 |         params->op = EC_VBNV_CONTEXT_OP_READ;
>       |               ^~
> In file included from drivers/platform/chrome/cros_ec_vbc.c:12:
> In function ‘kmalloc’,
>     inlined from ‘vboot_context_read’ at drivers/platform/chrome/cros_ec_vbc.c:30:8:
> ./include/linux/slab.h:580:24: note: at offset 20 into object of size 36 allocated by ‘kmalloc_trace’
>   580 |                 return kmalloc_trace(
>       |                        ^~~~~~~~~~~~~~
>   581 |                                 kmalloc_caches[kmalloc_type(flags)][index],
>       |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   582 |                                 flags, size);
>       |                                 ~~~~~~~~~~~~
> 
> Link: https://github.com/KSPP/linux/issues/278
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

This patch seems to have gotten lost? Looking at the conversation, I
think it should land as-is rather than changing the allocation size.

I can pick this up via my tree if that helps...

-Kees

> ---
>  drivers/platform/chrome/cros_ec_vbc.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/platform/chrome/cros_ec_vbc.c b/drivers/platform/chrome/cros_ec_vbc.c
> index c859c862d7ac..b5a584f5469a 100644
> --- a/drivers/platform/chrome/cros_ec_vbc.c
> +++ b/drivers/platform/chrome/cros_ec_vbc.c
> @@ -20,10 +20,14 @@ static ssize_t vboot_context_read(struct file *filp, struct kobject *kobj,
>  	struct device *dev = kobj_to_dev(kobj);
>  	struct cros_ec_dev *ec = to_cros_ec_dev(dev);
>  	struct cros_ec_device *ecdev = ec->ec_dev;
> -	struct ec_params_vbnvcontext *params;
>  	struct cros_ec_command *msg;
> +	/*
> +	 * This should be a pointer to the same type as op field in
> +	 * struct ec_params_vbnvcontext.
> +	 */
> +	uint32_t *params_op;
>  	int err;
> -	const size_t para_sz = sizeof(params->op);
> +	const size_t para_sz = sizeof(*params_op);
>  	const size_t resp_sz = sizeof(struct ec_response_vbnvcontext);
>  	const size_t payload = max(para_sz, resp_sz);
>  
> @@ -32,8 +36,8 @@ static ssize_t vboot_context_read(struct file *filp, struct kobject *kobj,
>  		return -ENOMEM;
>  
>  	/* NB: we only kmalloc()ated enough space for the op field */
> -	params = (struct ec_params_vbnvcontext *)msg->data;
> -	params->op = EC_VBNV_CONTEXT_OP_READ;
> +	params_op = (uint32_t *)msg->data;
> +	*params_op = EC_VBNV_CONTEXT_OP_READ;
>  
>  	msg->version = EC_VER_VBNV_CONTEXT;
>  	msg->command = EC_CMD_VBNV_CONTEXT;
> -- 
> 2.34.1
> 

-- 
Kees Cook